1. Field of the Invention
This invention pertains generally to separating fluids, and more particularly to separating and recovering viscous fluids from water or other fluids.
2. Description of Related Art
Mechanical recovery is the most commonly used oil spill response technique, and is a technique that is used in industrial applications as well. This technique physically removes oil from the water surface, and the oil is usually floating on the water surface. Unlike other cleanup techniques, mechanical recovery can be efficiently applied to treat emulsified oils as well as oils of variable viscosities. The main weakness of mechanical cleanup is the recovery rate. Mechanical recovery may be very time consuming and expensive when employed on a large scale. Mechanical recovery may also require a large number of personnel and equipment, and every additional hour of cleanup time can significantly increase the cost of recovery. Therefore, a more efficient recovery device could reduce cleanup costs significantly, as well as reduce the risk of oil reaching the shoreline.
An adhesion (oleophilic) skimmer is one of the most common types of mechanical recovery equipment. This type of skimmer is based on the adhesion of oil to a rotating skimmer surface. The rotating surface lifts the oil out of the water to an oil removal device (e.g., scraper, roller, etc.). The adhesion surface is the most critical element of the skimmer as it determines the efficiency of recovery. Various shapes of the skimmer, such as a mop, belt, brush, disc, and drum, have been developed to increase skimmer efficiency.
Two types of recovery surfaces patterns are usually used for adhesion oil skimmers. Smooth flat surfaces are used on drum, disk and belt skimmers. Drum and belt skimmers might also have a surface covered with brushes. The latter configuration has an obvious advantage due to the much higher surface area (oil covering every bristle) and formation of oil meniscuses between the bristles, but the difficulty of oil removal from the brushes may result in a lower overall recovery. Brush surfaces tend to pick up debris and water together with oil, which may affect the recovery efficiency and oil transfer process. The smooth surface area of a drum, disk and belt doesn't usually recover debris, but this configuration picks up less oil than a brush surface due to the smaller surface area.
The oil spill recovery process has two equally important goals. The first one is to remove oil from the water surface and the second one is to remove oil adhered to the recovery surface and transfer it into the collector. The recovery efficiency depends on the achievement of both of these goals. In the case of a smooth surface, the amount of recovered oil is relatively low, but close to 100% of it can be removed by a scraper. In the case of a brush surface and light to medium oils, oil covers every bristle and forms small menisci between the bristles, preventing oil from draining back into the slick. Unfortunately, the configuration of this surface doesn't allow for scraping every bristle individually and removing all adhered oil. Hence, a significant amount of oil remains on the surface after scraping and returns back to the oil slick, thereby reducing the overall recovery rate.
A brush configuration works much more efficiently on high viscosity and semi-solid oils. In this case, oil doesn't cover the bristles or penetrate inside the brush. It is merely being lifted from the water by the tips of the bristles and physically transported to the collector. This process is not exactly related to oil adhesion and spreading properties. This explains the ability of a brush surface to recover more debris than a smooth surface.
Accordingly, using brushes increases the contact surface area between oil and recovery device, and exploits the effect of capillary forces for collection of oil between the bristles. A disadvantage of the brush method, however, is the fact that brushes collect debris and water together with oil, which can clog the pipes in oil-collection device. Another disadvantage is its inability to remove large part of the oil adhered to brushes using scrapers, since they cannot scrape each brush individually. Improvement has been sought by using porous mats (or similar structures) covering the surface of the skimmer, allowing oil to penetrate into its matrix, be lifted from the water, and squeezed out by rollers into the collection device. However, such improvements are intended to increase the volume of oil that can be recovered from water per unit area of the recovery surface. Although such improvements allow a thicker oil film to be formed on the recovery surface, they do not allow for scraping out all of the recovered oil. In contrast, belts and drums with smooth surfaces allow almost 100% of adhered oil to be transferred into collector. The disadvantage of smooth recovery surfaces, however, is that only a relatively thin film can be formed on its surface and total volume of the recovered oil is relatively small.
To select the most efficient oil spill response action, it is important to understand the chemistry and physical behavior of spilled oil and the way these characteristics change over time. Viscosity increase and emulsion formation are dynamic processes of particular interest. Petroleum products and oils originated at different oil fields have extremely diverse properties and chemical compositions. Viscosity of these products can vary in the range of 0.5 mPas to 100,000 mPas. Oil weathering brings additional complication to the prediction of spilled oil properties and has significant ramifications with respect to appropriate recovery strategies. During the first twenty-four hours, some oils can lose from 5% to 50% of light compounds. A major increase in oil viscosity, caused by evaporation of lighter compounds and emulsification, will occur within hours to a few days. Therefore, the oil that has to be recovered does not have the same properties as the oil that has been spilled. Existing types of skimmers are not tailored to the properties of the product that has to be recovered and can only recover oil within a certain range of properties. They are characterized by a specific “window of opportunity”—a time period when this equipment may be used, which is largely determined by the oil properties (viscosity in particular). Outside of that time period, response measures with this equipment may become ineffective.